The present invention relates to gyrocompasses and, in particular, to methods for accurately determining north and direction.
It is known to use a gyroscope to determine north by sensing the rotation of the earth. A gyroscope having a horizontal spin axis responds to the horizontal component of the angular rate of rotation of the earth by producing a torque on the gyroscope tending to align its spin axis with that of the earth.
It is also known to use a gyroscope having a single degree of freedom so that only the horizontal component of the earth's rotation is influential. This degree of freedom is about a vertical axis. Since the torque produced by the rotation of the earth is rather small, known gyrocompasses employ an air bearing to substantially eliminate resisting torques.
It is also known to rotate the input axis of a dualfunction gyroscope 90.degree. and thereby shift its degree of freedom from a horizontal to a vertical axis, for example, U.S. Pat. No. 3,254,419. This 90.degree. rotation shifts the operation from directional to north seeking. A gyroscope of the latter type, however, does not employ a torquer to affect motion about the single degree of freedom of the gyroscope. Also in gyrocompasses of this type, the large amounts of azimuthal rotation required to swing the gyroscope towards north is accomplished by a servo which supports a pitch and roll gimbal. Consequently, the azimuthal motor is required to carry two additional gimbals mechanisms as well as the central gyroscope.
A similar, known gyroscope (U.S. Pat. No. 3,394,596) is so mounted within an azimuthally rotatable gimbal that the degree of rotational freedom of the gyroscope can also be shifted from a horizontal to a vertical axis. Again, this type of dual-function gyroscope does not employ a torquer to restrain motion about the single degree of freedom of the gyroscope. Accordingly, this gyroscope does not measure the horizontal component of the rate of angular rotation of the earth transverse to the spin axis of the gyroscope (north seeking mode). Consequently, this type of gyroscope is unable to estimate the extent to which its spin axis is misaligned with respect to north. Also, this known gyroscopic device employs a follow-up means which operates only in the north seeking mode. The follow-up is mechanically disengaged when the gyro is swung into its directional position. Such mechanical disengagement complicates the gyroscope and tends to unbalance the central gyro by necessitating a motor or a pinion nonconcentrically mounted within the device.
Accordingly, there is a need for a gyroscopic system which can accurately determine north and avoids the errors of the systems of the prior art. There is also a need for a gyrocompass that can be shifted to operate in a directional mode.